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Limestone, pH and calcium

Craig Dick Published on 01 May 2014

Growing 5-ton hay? Then your crop will remove 62.5 pounds phosphorus (P), 250 pounds potassium (K), 150 pounds calcium (Ca), 30 pounds magnesium (Mg), 30 pounds sulfur (S) and 0.4 pounds boron (B).

One of the most common points of confusion concerns limestone, pH and calcium. Many think these refer to the same thing. There is a big difference in pH and calcium, as well as limestone and limestone sources.

Knowing the difference can mean big yield and quality increases in your forages.

Limestone is mainly used in agriculture to regulate soil acidity, but highly reactive calcitic lime can also be a source of calcium, the most important nutrient for soil and plant health.

How soil becomes acidic
There are four basic ways soils become acidic:

1. Some soils are naturally formed from parent material low in carbonates (both calcium and magnesium), and they are usually acidic.

2. Soils that form under high rainfall are subject to extensive leaching and weathering, and allow acidic cations (H+ and Al3+) to occupy the empty soil exchange sites.

3. Harvesting crops removes basic cations (positively charged ions), increasing soil acidity.

4. Ammoniacal fertilizers convert in the soil to nitrate, creating acidic soil.

Soil pH – active acidity
Simply stated, pH is the acidity or alkalinity of any solution; it’s merely a measurement of hydrogen in the soil (H+). As the hydrogen ion concentration increases, the resulting pH number decreases.

The pH scale ranges from 1 to 14, with 7 being neutral. Each numerical step in the pH scale represents a tenfold increase or decrease in acidity. So pH 5 is 10 times more basic than pH 6 and 100 times more acidic than pH 7.

Calcium has nothing to do with the pH, it is merely a measurement of hydrogen.

Buffer pH
Active acidity accounts for the H+ ions in the soil/water solution and doesn’t account for the reserve, or potential acidity, reported as buffer pH.

Soils vary in their ability to resist change in pH. If a soil has a large amount of clay or organic matter, it will have a high cation exchange capacity (CEC), which is its ability to hold on to large amounts of cations (H, Ca, Mg, K, Na, etc.).

Soils with a low CEC are generally sandy in nature and lack the ability to hold onto cations. Soils with higher CEC are more resistant to quick change, and this is reflected in the buffer pH reading that shows up on your soil test.

The lower the buffer pH, the more liming material will be needed to correct the acidity.

Since the CEC of a soil has a large impact on what the buffer pH of the soil will be, the buffer pH and actual soil pH are determined when making a lime recommendation.

You should always consult with your lime dealer or agronomist to figure out your exact needs based on your soil test, soil type, starting pH and lime source.

How lime reduces soil acidity
The secret to limestone’s effectiveness is not how much is applied but how much lime dissolves in the soil solution, which is where the neutralization process occurs. Once limestone is dissolved in the soil solution, the carbonate molecule is available to neutralize acidity.

The carbonate molecule (CO3--) reacts with two hydrogen ions (H+) on the soil exchange site. This process creates more water (H2O) and carbon dioxide (CO2).

As the H+ ions are converted to H2O, soil acidity is neutralized and one calcium ion (Ca++) replaces two H+ ions on the soil exchange site.

Lime

In the picture, a highly reactive lime (left) and slower-reacting lime (right) were added to water that had been acidified to pH 2.8. As acid is neutralized, carbon dioxide (CO2) is given off.

Acid neutralization is evident in the picture, with the effervescent head on the beaker containing a highly reactive lime (left). This is the indicator of how complete and rapid limestone is working to change pH.

Remember: The calcium does not affect the pH change, it is the carbonate molecule that does the work.

How to find a highly reactive limestone
Source, purity and fineness of grind strongly impact the reactivity of a liming material. Most limestone is either not pure enough or not crushed fine enough to be considered highly reactive.

Note: There are multiple ways to score lime across the U.S., and many states use different fineness factors to assign an overall quality score.

Purity – calcium carbonate equivalent (CCE)
The purity of limestone is expressed as calcium carbonate equivalent (CCE). It is a laboratory measure of how much calcium carbonate reacts in a given sample. Limestone should have a CCE of at least 90 percent.

Liming material CCE can range from 50 to 175 percent. Much of the country’s ag lime contains many impurities – such as sand, clay, iron and lead – which lower its CCE.

Fineness of grind – effective calcium carbonate equivalent (ECCE)
The effective calcium carbonate equivalent (ECCE) is a measure of the limestone’s effectiveness and is based on the combined effect of chemical purity (CCE), fineness of grind and moisture content.

Limestone effectiveness

Table 1 shows limestone effectiveness based on being ground and sieved through different-sized mesh screens. It also can be referred to as effective neutralizing value (ENV), total neutralizing power (TNP), effective neutralizing material (ENM) and in one state as the “lime score.”

Even very pure calcium carbonate (limestone) is not very soluble in water, so it must be finely ground to effectively neutralize soil acid. While grinding finer increases effectiveness, this also increases cost and drift loss.

To avoid drift, many quarries will only grind limestone to an average of 30 mesh. However, lime that is not ground to at least 30 mesh will generally not make a significant impact on soil pH.

Stone to acid soil

What about dolomitic lime?
Calcium magnesium carbonate (dolomite) is often used for liming. Dolomite supplies magnesium, a consideration if you are trying to increase magnesium content, but should be otherwise avoided.

While dolomitic lime tends to get a higher CCE score, dolomite is less soluble than calcium carbonate and reacts more slowly, resulting in a lower soil pH change.

Calcium
While pH has nothing to do with calcium, a highly reactive calcitic limestone can provide plant-available calcium. Calcium should be considered the most important nutrient.

It plays a major role in the physiology of the plant, strengthening its physical structure, increasing nutrient uptake and protecting from stress.

Calcium in the soil reduces soil compaction, increases water infiltration and helps to provide a better environment for the proliferation of beneficial bacteria.

Calcium is an important nutrient; only nitrogen and potassium are required in larger amounts by plants. Over-fertilization of nitrogen can leach calcium, and excess potassium application will reduce calcium availability.

A common misconception is that soils contain adequate calcium and there is no need to apply calcium. However, calcium in the soil is relatively insoluble, and forages like alfalfa respond well to applications of calcium.

Understanding the difference between limestone sources, what pH is and how it is different from calcium will ensure high-quality, high-yielding forages.

Studies conducted at the University of Wisconsin have proven that fixing acidic soils can increase yields by 40 percent. Choosing a highly reactive limestone is the key to fixing soil pH problems and adding profit to your bottom line.  FG

Craig Dick is a VP of sales and marketing with Calcium Products Inc.

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